1. Field of the Invention
The present invention relates to an apparatus and a method for controlling duty solenoid valves each of which controls the pressure or flow rate of a fluid.
2. Description of the Related Art
Conventionally, it is known a duty solenoid valve control apparatus described in Unexamined Japanese Patent Publication (Kokai) Hei-3-216713.
In the apparatus described in JP-A-3-216713, overexcitation control is performed so that a current rises rapidly in a solenoid coil when the solenoid coil is conducted to thereby open a duty solenoid valve rapidly, and holding control is performed after the overexcitation control so that the duty solenoid valve is held in an opened state with the current at an irreducible minimum value. FIGS. 9A to 9D are time charts showing this condition. In FIG. 9A, a duty signal has a pulse duration Tx in each pulse repetition period To so that the duty signal becomes high (H) in level in the pulse duration Tx. In this pulse duration Tx, a micro computer outputs an overexcitation signal having a predetermined pulse width in a period Tc, and in a remainder period of (Tx-Tc), the micro computer outputs a holding signal composed of chopping pulses. In this manner, the overexcitation signal and the holding signal constitute a pulse width modulation (PWM) signal as shown in FIG. 9B. The voltage and current in the solenoid coil in this condition have waveforms as shown respectively in the FIGS. 9C and 9D.
More specifically, the micro computer generates such a PWM signal as shown in FIG. 9B on the basis of the flow chart of FIG. 10 as follows. The micro computer executes an interruption process simultaneously with the rising of the duty signal to thereby output an overexcitation signal and then executes an interruption process again to thereby output a holding signal at the timing when the overexcitation control is to be switched over to the holding control. When the duty signal falls down, the micro computer executes an interruption process again to thereby cut off the signal supply to the duty solenoid valve.
Because the duty solenoid valve control apparatus described in JP-A-3-216713 operates as described above, an interruption process (FIG. 10) is required whenever any one of overexcitation control, holding control and cutting-off of a signal supplied to the duty solenoid valve is carried out.
Further, the signal supplied to the solenoid coil as shown in FIG. 9B is given to one duty solenoid valve or a plurality of duty solenoid valves of the identical operation.
Accordingly, if a plurality of duty solenoid valves different in operation from each other are to be controlled, the same number of interruption processes as the number of duty solenoid valves are required. Therefore, the program capacity must be large. Further, because the number of interruption processes increases, processing cannot be made at a high speed. There arises a problem that the duty solenoid valves cannot be driven at a high speed.
Further, if the number of duty solenoid valves increases, there arises a problem that the control apparatus becomes complex.